Question

Discuss the trend of stability of oxides, carbonates and sulphates of Group 2 elements.

Short answer

The stability of oxides and carbonates of group 2 elements increases as we move down the group, due to an increase in lattice energy and less polarization of anions by cations respectively. On the other hand, the stability of sulphates decreases as we move down the group, due to the reduction in lattice energy.

Step-by-step solution

Stability of Oxides

Firstly, consider the chemical nature of the group 2 elements and their oxides. Group 2 elements form oxides with formula MO, where M represents a Group 2 element. As we descend the group, the stabilization of the oxides increases due to the increase in lattice energy that comes from the smaller size of the oxides.

Stability of Carbonates

Next, the stability of the carbonates will be discussed - these compounds have the formula MCO3. The general trend in carbonates is that, like oxides, stability increases as we move down the group. This is due to the fact that the polarization of the larger anions (carbonate ions) by the smaller cations on top of the group is more, leading to the decomposition of the carbonate. As we move down the group, the cation size increases, which can’t polarize the anion. Thus, stability increases down the group.

Stability of Sulphates

Lastly, the stability of sulphates of Group 2 elements (MSO4 compounds) will be considered. Unlike the previous trends, sulphates become less stable as we move down the group. This is primarily due to the decrement in the lattice energy of the larger sulphate ions by the increasing size of the cations.

Key concepts

Oxides Stability Trend

Group 2 elements are known as alkaline earth metals and include beryllium, magnesium, calcium, strontium, and barium. When these elements form oxides, they create compounds with the formula MO, where M refers to the metal. The trend in the stability of these oxides increases as you move down the group from beryllium to barium. This is due to the increase in lattice energy, which stabilizes the crystal structure of the oxides. Lattice energy refers to the energy released when ions are packed together in a lattice.

Higher lattice energy in the oxides of heavier Group 2 elements occurs because their ions are larger, which makes the oxides more stable. The larger size of metal ions in the lower part of the group leads to a stronger lattice owing to the effective balance of electrostatic forces.

Students should note that this increased stability is a distinctive characteristic of Group 2 oxides, reflecting the greater thermodynamic favorability in their formation.

Carbonates Stability Trend

Carbonates of Group 2 elements, with the chemical formula MCO3, display a distinct trend in stability. The stability increases as one moves down the group from beryllium to barium. To understand this trend, it's crucial to consider the concept of polarization.

At the top of the group, smaller cations like beryllium and magnesium are highly polarizing, meaning they have the ability to distort the electron cloud of anions such as the carbonate ion. This polarization effect can lead to instability and decomposition.

However, as we move down the group, cations become larger and less polarizing, reducing this effect. The reduced polarization lessens the distortion of the anion's electron cloud, resulting in more stable carbonate compounds.

• For example, beryllium carbonate is the least stable, while barium carbonate is much more stable.

Thus, stability increment down the group can mainly be attributed to the increasing ionic radii which decreases polarization of the carbonate ion.

Sulphates Stability Trend

The stability of sulphates in Group 2 elements, represented by MSO4, differs from the trends observed in oxides and carbonates. As we descend the group, the stability of these sulphates actually decreases.

This seemingly reverse trend is connected to the lattice energy of the sulphates, which depends on the size of both the cation and anion. Sulphate ions themselves are relatively large, and when combined with the larger cations of lower Group 2 elements, the overall lattice becomes less strong.

A significant contributor to this trend is the decrease in hydration energy as the cation becomes larger. In general, smaller ions have higher hydration energy, which favors their solvation in water. As the cationic radius increases, the ability of water molecules to effectively interact with and stabilize the ion decreases, contributing to reduced stability.

• Magnesium sulphate remains relatively stable, while barium sulphate, on the other hand, can be less stable under similar conditions.

This means the decrease in lattice and hydration energies as we move down the group result in lower stability of sulphate compounds in Group 2.